1. Field of the Invention
The present invention relates to a snubber circuit for a power converter for reducing switching loss in the power converter.
2. Description of the Prior Art
FIG. 18 shows an example of a conventional snubber circuit employed in a voltage-dropping chopper circuit, a type of a power converter, wherein a DC power supply 1, has a first side of a switching element 4 connected to its positive power supply terminal 2. The switching element 4 uses an electronic switch such as an MOSFET, a transistor, an IGBT or a static induction transistor (SIT). A diode 5 is connected between the second side of the switching element 4 and a negative power supply terminal 3 of the DC power supply 1. A DC reactor 7 is connected to a connection point 6 of the second side of switching element 4 and the diode 5, which are connected in series. A load 8 is connected to the DC reactor 7 in a series circuit between terminal 3 and point 6. A conventional snubber circuit 12, comprising a capacitor 9 connected in series with the parallel arrangement of a resistor 10 and a diode 11, is connected across the connection point 6 and the power supply terminal 2.
FIGS. 19(a)-19(e) show waveform charts for illustrating the operation of the known snubber circuit employed in the voltage-dropping chopper circuitry in FIG. 18. FIG. 19(a) indicates ON/OFF operation of the switching element 4. FIG. 19(b) illustrates a voltage waveform thereof. FIG. 19(c) illustrates a current waveform thereof, FIG. 19(d) illustrates a current waveform of the resistor 10, and FIG. 19(e) illustrates a current waveform of the diode 5.
The operation of the conventional snubber circuit will now be described in accordance with FIGS. 19(a)-19(e). As shown in FIG. 19(a), when the switching element 4 is turned ON at time 80, a current which was flowing in a loop, consisting of the diode 5, the DC reactor 7 and the load 8, begins to flow in the load 8 through the switching element 4 via the DC reactor 7. At the instant 80 that the switching element 4 is switched from OFF to ON, a very large surge current caused by the recovery characteristic of the diode 5 flows in the diode 5 in an opposite direction as indicated by 81 in FIG. 19(e). Accordingly, as indicated by 82 in FIG. 19(c), a large surge current flows through the switching element 4 when the switching element 4 is turned ON.
The voltage of the connection point 6 rises when the switching element 4 is turned ON. As a result, a current flows in the resistor 10 through the capacitor 9. At this time, a current does not flow in the diode 11 since its direction is opposite. The current flowing in the resistor 10 is shown in FIG. 19(d). As illustrated, the peak value of the current flowing in the resistor 10 depends on the value of the resistor 10 and the voltage of the DC power supply 1, and the attenuation time constant of the current depends on the resistor 10 and the capacitor 9. Namely, the attenuation time constant is determined so that the capacitor 9 discharges while the switching element 4 is ON. The capacity of the capacitor 9 depends on the rise ratio (dv/dt) of the switching element 4 voltage indicated by 83 in FIG. 19(b) at a time when the switching element 4 is turned OFF. Hence, as the operating frequency of the voltage-dropping chopper circuitry is higher in frequency, the resistor 10 value becomes smaller. As in a current waveform 82 of the switching element 4 shown in FIG. 19(c), therefore, when the switching element 4 is switched ON, a sum of the recovery surge Current 81 of the diode 5, a current 84 flowing in the resistor 10 of the snubber circuit 12, and a current 85 flowing in the DC reactor 7 flows in the switching element 4. Moreover, the ON voltage of the switching element 4 is relatively high as indicated by 89 in FIG. 19(b) and there is a period when both the current and voltage are high.
As shown in FIG. 19(a), when the switching element 4 is switched OFF at time 86, the voltage of the switching element 4 attempts to rise, but the current flows through the diode 11 and the capacitor 9 of the snubber circuit 12 and restricts the voltage rise ratio (dv/dt) of the switching element 4 as indicated by 83 in FIG. 19(b). This immediately causes the switching element 4 current to be zeroed as indicated by 87 in FIG. 19(c). Since the switching element 4 voltage is zero at this time, the switching loss of the switching element 4 does not occur. When the voltage of the capacitor 9 has reached that of the DC power supply 1, the diode 5 is switched ON as indicated by 88 in FIG. 19(e) and the current keeps flowing in the DC reactor 7.
FIG. 20 shows an example of a conventional snubber circuit used in a half-bridge inverter circuit, a type of a power converter, wherein a load 8 is connected between a connection point 6 in a series circuit, comprising two switching elements 31 and 32 which are connected in parallel with a series circuit of DC power supplies 29, 30, and a connection point 33 of the DC power supplies 29, 30. Further, a snubber circuit 20, comprising a capacitor 21, a resistor 22 and a diode 23, is connected in parallel with the switching element 31, and also a snubber circuit 25, comprising a capacitor 26, a resistor 27 and a diode 28, is connected in parallel with the switching element 32.
When the switching element 32 is switched OF and the switching element 31 is switched ON in the half-bridge inverter circuitry shown in FIG. 20, a current which was flowing in the switching element 32 and the load 8 then begins to flow in the load 8 through the switching element 31. At this time, a very large surge current flows due to the recovery characteristic of the switching element 32 or of an opposite-direction diode (not illustrated) externally installed. In addition, the switching element 31 switched ON forces the voltage of the connection point 6 to rise and a current to flow in the resistor 22 through the capacitor 21. At this time, the current does not flow in the diode 23 because it is connected in the opposite direction. Further, a current flows in the diode 28 through the capacitor 26. Since this circuit does not have a resistor component, a large current will flow if the switching speed is high.
As described above, when the switching element 31 is switched ON, a sum of the recovery surge current of the switching current 32, the current flowing in the capacitor 26 and the diode 28 of the snubber circuit 25, the current flowing in the capacitor 21 and the resistor 22 of the snubber circuit 20, and the current flowing in the load 8 flows in the switching element 31. Accordingly, a very large surge current flows when the switching element 31 is switched ON. Similarly, a very large surge current also flows when the switching element 32 is switched ON. Moreover, since the currents are large, the ON voltages of the switching elements 31, 32 are high and there is a period when both the current and voltage are high.
In the conventional snubber circuits for a power converter designed as described above, for example, the snubber circuit 12 shown in FIG. 18, the switching element 4 experiences an extremely high switching loss. This loss is due to the fact that the sum of (i) the recovery surge current of the diode 5, (ii) the current flowing in the resistor 10 of the snubber circuit 12, and (iii) the current flowing in the DC reactor 7, flows in the switching element 4 when the switching element 4 is turned ON. Also, the ON voltage of the switching element 4 is comparatively high, and there is a period when both the current and voltage are high. In addition, because of the high recovery current flowing in the diode 5, the snubber circuit 12 also has a large switching loss at the diode 5 and the current flowing in the resistor 10 all turns into heat (loss). Concerning the snubber circuits 20, 25 shown in FIG. 20, since very large surge currents flow and the ON voltages are high when the switching elements 31, 32 are turned ON, the switching loss is very large and the currents flowing in the resistors 22, 27 all change into heat (loss). Accordingly, a power converter employing these snubber circuits will be low in efficiency.